A world without males: How the elm zigzag sawfly is benefiting from their absence

After zigzagging its way across most of Europe for 15 years, elm zigzag sawfly (Aproceros leucopoda) has now been confirmed in Britain following a discovery of the distinctive feeding traces in Surrey in 2017. Soon after a press release documenting the finding, and the significant media interest that followed, records of the sawfly were received via Tree Alert and the first record of the lava in Britain made on 24 June 2018.

The speed of elm zigzag’s spread is partially due to its frenetic lifestyle. In good weather, they can complete their life cycle in under a month, completing multiple generations before sheltering in a special, solid walled cocoon over winter. As well as growing at speed, the adult sawflies have another strategy for rapid colonisation – they don’t need to mate to lay fertile eggs.

Normally, sexually mature female animals need to mate with sexually mature males to create fertile offspring. However, in some species, females don’t need to mate to produce viable offspring. The embryos, instead, developing without fertilisation (known as parthenogenesis). There are a number of mechanisms by which this can take place. The specific one that elm zigzag sawfly employs is known as thelytoky (from the Greek meaning ‘female birth’). In this type of parthenogenesis, female sawflies are produced from unfertilised eggs. How exactly this takes place at the genetic level isn’t understood in elm zigzag sawfly, but the impact of this evolutionary strategy is considerable.

In evolutionary biology, there is an important cost associated with having sexual reproduction, known since the 1970’s as ‘the cost of males’. If, as in many sexually reproducing organisms, 50% of the offspring are male, then by only producing females there will be twice as many females able to lay eggs in the next generation. This results in the population being able to increase at a higher rate.

Not having to find a mate also saves time: females can just get straight on with laying eggs instead of having to waiting to find a suitable suitor. Being parthenogenic and able to complete multiple generations in a year allows just a single elm zigzag sawfly female to quickly establish a population in an area. We have no idea when elm zigzag sawfly first made it into the UK, but the current known distribution (as at 11/07/2018) of the sawfly suggests it has established itself over a wide area. But this is likely to be an underestimate of its true distribution.

Some areas of Europe have recorded elm zigzag sawflies as causing severe defoliation on elms – over 75% leaf loss. But, as of yet, nothing this serious has been seen in the UK. Elms can survive a single defoliation as long as they are not stressed by other factors. However, repeated defoliations may deplete their resources, eventually killing them. Though generally, thanks to Dutch elm disease in the UK, elm still supports a large number of specialist leaf feeding insects – most notably the white-letter hairstreak butterfly (Satyrium w-album), which may experience significant competition from elm zigzag sawfly.

As we are in the earliest phases of population growth and dispersal for elm zigzag, we would welcome reports of the species from areas (not noted on the map shown) and any records of the species causing significant amounts of damage to elms. These can all be submitted through Tree Alert.

Though identifying the feeding traces is generally straightforward, more photographs and comments on identification can be found on Forest Research’s webpage on the elm zigzag sawfly.

With your help we can get a better understanding of elm zigzag sawfly in the UK and can collect crucial data on its distribution. This can give us a baseline against which to compare any future population expansions the species undergoes.